The invention disclosed and claimed herein generally pertains to electromagnetic flowmeters, i.e., devices for measuring the velocity of a current present in a body of liquid or other fluid, wherein a permanent magnet is employed to generate the magnetic field required for flowmeter operation. More particularly, the invention pertains to electromagnetic flowmeters which employ a permanent magnet and which further employ a selectively activated electromagnet to determine a potential difference error term between the electrodes of the meter. Even more particularly, the invention pertains to flowmeters of the above type which minimize power required for the operation thereof, to enable unattended long term deployment in a body of selected fluid.
It is now well known that the velocity V of a current present in a body of conductive fluid may be readily determined by (1) inserting a pair of electrodes in the current, and (2) generating a magnetic field B, the electrodes being spaced apart by a distance d, and there being a potential difference E therebetween. From an application of Faraday's law, e.sub.x =V.sub.y B.sub.z d, where e.sub.x, V.sub.y and B.sub.z are mutually orthogonal components of the vectors E, V and B, respectively. E and B quantities may be readily measured by means of conventional instruments, to enable determination of current velocity.
Devices structured according to the above principles are generally known as electromagnetic flowmeters. In the past, such devices have been used to substantially improve sensitivity in current velocity measurement. Over the years, Applicant has made significant contributions to the electromagnetic flowmeter art, as indicated, for example, by patents granted to him by the United States Patent and Trademark Office for "Electromagnetic Flow Meter", U.S. Pat. No. 3,693,440 issued Sept. 26, 1972, and for "High Sensitivity Flowmeter", U.S. Pat. No. 4,000,648, issued Jan. 4, 1977.
There is growing interest in monitoring currents present in ocean or other salt water bodies, such as bottom currents. For such studies, it is necessary to deploy monitoring instruments at locations which are very remote, and for periods of time on the order of months, or in excess of a year. It is clearly important that the power requirements of monitoring equipment must be minimized, to enable a battery or other power source to provide sufficient power over the deployment period.
An electromagnetic flowmeter is a very useful device for monitoring current velocity. However, in the past, the magnetic field thereof has generally been generated by means of an electromagnetic coil, requiring a power on the order of 10 mw-1 watt. To eliminate a power drain of such magnitude, a flowmeter could alternatively employ a permanent magnet having the capability to provide a sufficient magnetic field for the entire period of deployment. However, when a pair of electrodes of a flowmeter are immersed into a conductive fluid, a potential difference occurs therebetween due to electrical interaction between the electrodes and the fluid. If the magnetic field of a flowmeter is AC, the effects of such potential difference are cancelled out. However, for the DC field provided by a permanent magnet, the relationship between current velocity, magnetic field, and potential difference between electrodes becomes e=VBd+e.sub..delta., where e.sub..delta. is the potential difference due to electrical interaction, hereinafter referred to as an error term. It is clear that velocity V cannot be determined without knowing the value of e.sub..delta..
The Applicant has discovered that by selectively combining a permanent magnet and an electromagnetic coil in an electromagnetic flowmeter, it becomes possible to determine the error term e.sub..delta. while limiting the power requirement of the flowmeter to an amount which is negligible in comparison with the aforementioned range of 10 mw-1 watt. Applicant thereby provides an electromagnetic flowmeter which is well suited for monitoring current velocity at a remote location in a selected fluid body for long-term periods. However, the invention disclosed and claimed herein is not intended to be limited to apparatus or structure intended for such purpose.